Since ZSM-5 having high silica content has been developed for the first time by Mobil Co. in the early 1970s, intensive research on such a material has been performed, due to its unique catalytic activity and shape selectivity resulting from the characteristics of ZSM-5 as a molecular sieve.
Unlike conventional alumino-silicate zeolites, ZSM-5 is generally prepared using various types of organic materials as a templating agent. Among organic materials known to be effective for templating ZSM-5 structure, tetrapropylammonium cation has been known the most effective. In practice, commercial ZSM-5, currently available, has been synthesized using such a tetrapropylammonium cation. However, although tetrapropylammonium has excellent template effects, research into the preparation of ZSM-5 without the use of such an organic template has been conducted. As a result, some preparation processes were developed.
The reason why the organic template is not used in the synthesis of ZSM-5 is expensive and very toxic, which can contaminate the environments. When ZSM-5 is synthesized using the organic template, secondary costs for treating a toxic organic material contained in unreactants are required. Also, the dangers of environmental contamination become very high.
In addition, ZSM-5 prepared by use of the above organic material should be subjected to a calcining step at 550° C. to pyrolytically remove the organic material present in channel structure of the ZSM-5, before being used as a catalyst. However, when the organic material is removed by the calcining step, the incomplete pyrolysis thereof results in pore blockage of ZSM-5, thus drastically decreasing the activity of the catalyst. Further, the use of the organic template is disadvantageous in terms of additional costs due to the calcining step, and air contamination by gases discharged upon pyrolysis of the organic material.
To overcome the above problems, in U.S. Pat. No. 4,257,885 (1981) to Flanigen et al., there is disclosed a method of synthesizing ZSM-5 with or without the use of a crystallization seed in the absence of an organic material. However, the above method has a drawback in that a reaction period is 68-120 hours.
Further, U.S. Pat. No. 4,565,681 (1986) to Kuhl discloses a method of synthesizing ZSM-5 at 150-200° C. for 8-48 hours by mixing a silica source with an acid-treated alumina source in the absence of an organic material. Furthermore, U.S. Pat. No. 5,240,892 (1993) to Klocke discloses a method of synthesizing ZSM-5 from a silica precursor neutralized with sulfuric acid in the absence of an organic template. However, the above methods have only 75% crystallinity, in spite of the reaction occurring at relatively high temperatures of 220° C. by using a crystallization seed acting to promote the crystallization.
Likewise, U.S. Pat. No. 5,254,327 (1993) to Martinez et al. discloses a method of synthesizing ZSM-5 by dissolving sodium aluminate in caustic soda without the use of a crystallization seed in the absence of an organic template, to prepare an aqueous solution, which is then mixed with colloidal silica However, this method requires a reaction period not less than 48 hours.
As mentioned above, the conventional methods of synthesizing ZSM-5 in the absence of the organic template are summarized by using the crystallization seed for promoting the crystallization, or neutralizing the alumina source with an acid solution to form a proper gel precursor, but have the disadvantage of a lengthy reaction period.